I always use apt full-upgrade. It definitely removed libwadseeker and installed libwadseeker1. Maybe some residual from development, but yeah I just need to regen the packages with libwadseeker1 installed and it'll be good.

$ sudo apt install libwadseeker
[sudo] password for wub:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
libwadseeker : Depends: libwadseeker1 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
$ sudo apt install libwadseeker1
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
giblib1 libqt5qml5 libqt5quick5 qml-module-qtgraphicaleffects qml-module-qtquick-controls qml-module-qtquick-layouts
qml-module-qtquick-window2 qml-module-qtquick2
Use 'sudo apt autoremove' to remove them.
The following NEW packages will be installed:
libwadseeker1
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 168 kB of archives.
After this operation, 1117 kB of additional disk space will be used.
Get:1https://debian.drdteam.org [^] stable/multiverse amd64 libwadseeker1 amd64 1.2 [168 kB]
Fetched 168 kB in 1s (196 kB/s)
Selecting previously unselected package libwadseeker1.
(Reading database ... 138329 files and directories currently installed.)
Preparing to unpack .../libwadseeker1_1.2_amd64.deb ...
Unpacking libwadseeker1 (1.2) ...
Setting up libwadseeker1 (1.2) ...
Processing triggers for libc-bin (2.27-6) ...
$ doomseeker
[22:26:09] Starting Doomseeker. Hello World! :)
# ...